I suspect the support costs are greater than the advantages to the provider. And, in most cases, it's not like there are a lot of choices of ISP. These days also a lot of choices of free providers for a simple static website--GitHub, pages on Google Blogger, etc.
I have my email grandfathered in by my ISP (Virginmrdia) but they don’t offer it to new customers.
I use it with my own domain name that redirects to the Virginmedia box, so not tied to them.